Well... it's not impossible. But as long as WoW introduces new content, it seems unlikely since they're likely to keep story progression in WoW. After WoW reaches its end, then we might see a Warcraft game. Then again, they might just retire the game line. Who knows? And then again, they could easily make "prequels" if they wanted to.
Well WC4 could be the lore storyline progression between expansion content. I mean technically it could be like The Shattering is to cataclysm that is between Wrath and cata and explains how certain things are the way they are..Thats just my 2 cents though o.o
Ya they will either do the content that we are now doing.
Do stuff before or in between warcraft(s).
Wait untill the stop updating wow and continue 4 from where we left off.
Stop warcraft since they still have starcraft as a rts.